Asked by: Michel Parent

As for how to see what the critical density is, we should turn to the Friedmann equation:

This equation determines how the scale factor of the universe (essentially the size of the universe), R, changes with time in terms of the energy density, , and k, a measure of the curvature of the universe. The first term in this equation is just the square of the Hubble parameter, H. We now just rearrange this equation:

This is how the critical density is defined. To calculate it, you just need to measure the Hubble parameter H and Newton's constant G. Currently, the best known values for H and G give a value for the critical density of about 1x10

Answered by: Andreas Birkedal-Hansen, M.A., Physics Grad Student, UC Berkeley

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